This invention relates generally to phototransistors and more specifically to phototransistors designed for a high gain response. In greater specificity, but without limitation thereto, the invention pertains to a phototransistor that utilizes both lateral and vertical bipolar transistor structures.
A phototransistor is similar to a bipolar transistor in that it has an emitter, base, and collector. While the base current of a bipolar transistor is provided through an electrical contact, the base current of a phototransistor, as its name suggests, is provided through photons.
Because light must be coupled into the base of the phototransistor, the phototransistor must be fabricated so that its base region is adjacent to the surface of the semiconductor. A typical such structure is referred to as a xe2x80x9clateralxe2x80x9d bipolar phototransistor, and is illustrated in FIG. 1. Photonic energy is absorbed at the surface of the base of this transistor. In distinction, a typical xe2x80x9cverticalxe2x80x9d bipolar transistor has a base region within the bulk of the semiconductor. This vertical design is illustrated in FIG. 2.
All semiconductors experience an increase in electron and hole (carrier) recombination near the semiconductor""s surface. Such activity reduces carrier lifetime, and thereby, the gain of the transistor.
In manufacturing, the base width of vertical transistors can be precisely made, as this width is controlled by how deep impurities are implanted or how far they are diffused into the semiconductor. In a lateral transistor, however, the base width must be controlled by lithography, a more difficult and less precise process. By combining lateral and vertical structures in a phototransistor design, the advantages of both structures can be had and a high gain phototransistor can be realized.
The invention provides a high gain phototransistor incorporating the advantages of lateral and vertical bipolar transistor structures. The invention is configured with a triple base, wherein the base regions of the vertical transistor structures are provided in the bulk of a semiconductor substrate while the base region of the lateral transistor structure is made adjacent to a surface of the phototransistor. Light is received on this surface.
Minority carrier generation is not limited to the base region of the lateral transistor but also extends to the base regions of the vertical transistors. Minority carriers will be present in these vertical base regions within a diffusion length of the optically generated carriers of the lateral transistor base.
From an electrical perspective, the bases of all three transistor structures are electrically connected. The collector electrodes of one of the vertical structures and the lateral structure are electrically connected, while the emitter electrodes of the other of the vertical structures and the lateral structures are electrically connected. Finally, the remaining vertical collector and emitter electrodes are electrically connected via the buried layer adjacent the phototransistor wafer substrate.
An object of this invention is to provide a phototransistor of relatively high gain.
Another object of this invention is to provide a phototransistor that provides the benefits of lateral and vertical transistor structures.
Still another object of the invention is to provide a triple-base phototransistor.
Other objects, advantages and new features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawing.